'\" te
.\" Copyright (c) 2013 by Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
.\" Copyright (c) 2017 Datto Inc.
.\" Copyright (c) 2018 by Delphix. All rights reserved.
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.\"
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.TH ZFS-MODULE-PARAMETERS 5 "Oct 28, 2017"
.SH NAME
zfs\-module\-parameters \- ZFS module parameters
.SH DESCRIPTION
.sp
.LP
Description of the different parameters to the ZFS module.

.SS "Module parameters"
.sp
.LP

.sp
.ne 2
.na
\fBdbuf_cache_max_bytes\fR (ulong)
.ad
.RS 12n
Maximum size in bytes of the dbuf cache.  When \fB0\fR this value will default
to \fB1/2^dbuf_cache_shift\fR (1/32) of the target ARC size, otherwise the
provided value in bytes will be used.  The behavior of the dbuf cache and its
associated settings can be observed via the \fB/proc/spl/kstat/zfs/dbufstats\fR
kstat.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBdbuf_metadata_cache_max_bytes\fR (ulong)
.ad
.RS 12n
Maximum size in bytes of the metadata dbuf cache.  When \fB0\fR this value will
default to \fB1/2^dbuf_cache_shift\fR (1/16) of the target ARC size, otherwise
the provided value in bytes will be used.  The behavior of the metadata dbuf
cache and its associated settings can be observed via the
\fB/proc/spl/kstat/zfs/dbufstats\fR kstat.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBdbuf_cache_hiwater_pct\fR (uint)
.ad
.RS 12n
The percentage over \fBdbuf_cache_max_bytes\fR when dbufs must be evicted
directly.
.sp
Default value: \fB10\fR%.
.RE

.sp
.ne 2
.na
\fBdbuf_cache_lowater_pct\fR (uint)
.ad
.RS 12n
The percentage below \fBdbuf_cache_max_bytes\fR when the evict thread stops
evicting dbufs.
.sp
Default value: \fB10\fR%.
.RE

.sp
.ne 2
.na
\fBdbuf_cache_shift\fR (int)
.ad
.RS 12n
Set the size of the dbuf cache, \fBdbuf_cache_max_bytes\fR, to a log2 fraction
of the target arc size.
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBdbuf_metadata_cache_shift\fR (int)
.ad
.RS 12n
Set the size of the dbuf metadata cache, \fBdbuf_metadata_cache_max_bytes\fR,
to a log2 fraction of the target arc size.
.sp
Default value: \fB6\fR.
.RE

.sp
.ne 2
.na
\fBignore_hole_birth\fR (int)
.ad
.RS 12n
When set, the hole_birth optimization will not be used, and all holes will
always be sent on zfs send. Useful if you suspect your datasets are affected
by a bug in hole_birth.
.sp
Use \fB1\fR for on (default) and \fB0\fR for off.
.RE

.sp
.ne 2
.na
\fBl2arc_feed_again\fR (int)
.ad
.RS 12n
Turbo L2ARC warm-up. When the L2ARC is cold the fill interval will be set as
fast as possible.
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE

.sp
.ne 2
.na
\fBl2arc_feed_min_ms\fR (ulong)
.ad
.RS 12n
Min feed interval in milliseconds. Requires \fBl2arc_feed_again=1\fR and only
applicable in related situations.
.sp
Default value: \fB200\fR.
.RE

.sp
.ne 2
.na
\fBl2arc_feed_secs\fR (ulong)
.ad
.RS 12n
Seconds between L2ARC writing
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBl2arc_headroom\fR (ulong)
.ad
.RS 12n
How far through the ARC lists to search for L2ARC cacheable content, expressed
as a multiplier of \fBl2arc_write_max\fR
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBl2arc_headroom_boost\fR (ulong)
.ad
.RS 12n
Scales \fBl2arc_headroom\fR by this percentage when L2ARC contents are being
successfully compressed before writing. A value of 100 disables this feature.
.sp
Default value: \fB200\fR%.
.RE

.sp
.ne 2
.na
\fBl2arc_noprefetch\fR (int)
.ad
.RS 12n
Do not write buffers to L2ARC if they were prefetched but not used by
applications
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE

.sp
.ne 2
.na
\fBl2arc_norw\fR (int)
.ad
.RS 12n
No reads during writes
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBl2arc_write_boost\fR (ulong)
.ad
.RS 12n
Cold L2ARC devices will have \fBl2arc_write_max\fR increased by this amount
while they remain cold.
.sp
Default value: \fB8,388,608\fR.
.RE

.sp
.ne 2
.na
\fBl2arc_write_max\fR (ulong)
.ad
.RS 12n
Max write bytes per interval
.sp
Default value: \fB8,388,608\fR.
.RE

.sp
.ne 2
.na
\fBmetaslab_aliquot\fR (ulong)
.ad
.RS 12n
Metaslab granularity, in bytes. This is roughly similar to what would be
referred to as the "stripe size" in traditional RAID arrays. In normal
operation, ZFS will try to write this amount of data to a top-level vdev
before moving on to the next one.
.sp
Default value: \fB524,288\fR.
.RE

.sp
.ne 2
.na
\fBmetaslab_bias_enabled\fR (int)
.ad
.RS 12n
Enable metaslab group biasing based on its vdev's over- or under-utilization
relative to the pool.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBmetaslab_force_ganging\fR (ulong)
.ad
.RS 12n
Make some blocks above a certain size be gang blocks.  This option is used
by the test suite to facilitate testing.
.sp
Default value: \fB16,777,217\fR.
.RE

.sp
.ne 2
.na
\fBzfs_metaslab_segment_weight_enabled\fR (int)
.ad
.RS 12n
Enable/disable segment-based metaslab selection.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBzfs_metaslab_switch_threshold\fR (int)
.ad
.RS 12n
When using segment-based metaslab selection, continue allocating
from the active metaslab until \fBzfs_metaslab_switch_threshold\fR
worth of buckets have been exhausted.
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBmetaslab_debug_load\fR (int)
.ad
.RS 12n
Load all metaslabs during pool import.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBmetaslab_debug_unload\fR (int)
.ad
.RS 12n
Prevent metaslabs from being unloaded.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBmetaslab_fragmentation_factor_enabled\fR (int)
.ad
.RS 12n
Enable use of the fragmentation metric in computing metaslab weights.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBvdev_max_ms_count\fR (int)
.ad
.RS 12n
When a vdev is added target this number of metaslabs per top-level vdev.
.sp
Default value: \fB200\fR.
.RE

.sp
.ne 2
.na
\fBvdev_min_ms_count\fR (int)
.ad
.RS 12n
Minimum number of metaslabs to create in a top-level vdev.
.sp
Default value: \fB16\fR.
.RE

.sp
.ne 2
.na
\fBvdev_ms_count_limit\fR (int)
.ad
.RS 12n
Practical upper limit of total metaslabs per top-level vdev.
.sp
Default value: \fB131,072\fR.
.RE

.sp
.ne 2
.na
\fBmetaslab_preload_enabled\fR (int)
.ad
.RS 12n
Enable metaslab group preloading.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBmetaslab_lba_weighting_enabled\fR (int)
.ad
.RS 12n
Give more weight to metaslabs with lower LBAs, assuming they have
greater bandwidth as is typically the case on a modern constant
angular velocity disk drive.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBspa_config_path\fR (charp)
.ad
.RS 12n
SPA config file
.sp
Default value: \fB/etc/zfs/zpool.cache\fR.
.RE

.sp
.ne 2
.na
\fBspa_asize_inflation\fR (int)
.ad
.RS 12n
Multiplication factor used to estimate actual disk consumption from the
size of data being written. The default value is a worst case estimate,
but lower values may be valid for a given pool depending on its
configuration.  Pool administrators who understand the factors involved
may wish to specify a more realistic inflation factor, particularly if
they operate close to quota or capacity limits.
.sp
Default value: \fB24\fR.
.RE

.sp
.ne 2
.na
\fBspa_load_print_vdev_tree\fR (int)
.ad
.RS 12n
Whether to print the vdev tree in the debugging message buffer during pool import.
Use 0 to disable and 1 to enable.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBspa_load_verify_data\fR (int)
.ad
.RS 12n
Whether to traverse data blocks during an "extreme rewind" (\fB-X\fR)
import.  Use 0 to disable and 1 to enable.

An extreme rewind import normally performs a full traversal of all
blocks in the pool for verification.  If this parameter is set to 0,
the traversal skips non-metadata blocks.  It can be toggled once the
import has started to stop or start the traversal of non-metadata blocks.
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBspa_load_verify_metadata\fR (int)
.ad
.RS 12n
Whether to traverse blocks during an "extreme rewind" (\fB-X\fR)
pool import.  Use 0 to disable and 1 to enable.

An extreme rewind import normally performs a full traversal of all
blocks in the pool for verification.  If this parameter is set to 0,
the traversal is not performed.  It can be toggled once the import has
started to stop or start the traversal.
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBspa_load_verify_maxinflight\fR (int)
.ad
.RS 12n
Maximum concurrent I/Os during the traversal performed during an "extreme
rewind" (\fB-X\fR) pool import.
.sp
Default value: \fB10000\fR.
.RE

.sp
.ne 2
.na
\fBspa_slop_shift\fR (int)
.ad
.RS 12n
Normally, we don't allow the last 3.2% (1/(2^spa_slop_shift)) of space
in the pool to be consumed.  This ensures that we don't run the pool
completely out of space, due to unaccounted changes (e.g. to the MOS).
It also limits the worst-case time to allocate space.  If we have
less than this amount of free space, most ZPL operations (e.g. write,
create) will return ENOSPC.
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBvdev_removal_max_span\fR (int)
.ad
.RS 12n
During top-level vdev removal, chunks of data are copied from the vdev
which may include free space in order to trade bandwidth for IOPS.
This parameter determines the maximum span of free space (in bytes)
which will be included as "unnecessary" data in a chunk of copied data.

The default value here was chosen to align with
\fBzfs_vdev_read_gap_limit\fR, which is a similar concept when doing
regular reads (but there's no reason it has to be the same).
.sp
Default value: \fB32,768\fR.
.RE

.sp
.ne 2
.na
\fBzfetch_array_rd_sz\fR (ulong)
.ad
.RS 12n
If prefetching is enabled, disable prefetching for reads larger than this size.
.sp
Default value: \fB1,048,576\fR.
.RE

.sp
.ne 2
.na
\fBzfetch_max_distance\fR (uint)
.ad
.RS 12n
Max bytes to prefetch per stream (default 8MB).
.sp
Default value: \fB8,388,608\fR.
.RE

.sp
.ne 2
.na
\fBzfetch_max_streams\fR (uint)
.ad
.RS 12n
Max number of streams per zfetch (prefetch streams per file).
.sp
Default value: \fB8\fR.
.RE

.sp
.ne 2
.na
\fBzfetch_min_sec_reap\fR (uint)
.ad
.RS 12n
Min time before an active prefetch stream can be reclaimed
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_dnode_limit\fR (ulong)
.ad
.RS 12n
When the number of bytes consumed by dnodes in the ARC exceeds this number of
bytes, try to unpin some of it in response to demand for non-metadata. This
value acts as a ceiling to the amount of dnode metadata, and defaults to 0 which
indicates that a percent which is based on \fBzfs_arc_dnode_limit_percent\fR of
the ARC meta buffers that may be used for dnodes.

See also \fBzfs_arc_meta_prune\fR which serves a similar purpose but is used
when the amount of metadata in the ARC exceeds \fBzfs_arc_meta_limit\fR rather
than in response to overall demand for non-metadata.

.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_dnode_limit_percent\fR (ulong)
.ad
.RS 12n
Percentage that can be consumed by dnodes of ARC meta buffers.
.sp
See also \fBzfs_arc_dnode_limit\fR which serves a similar purpose but has a
higher priority if set to nonzero value.
.sp
Default value: \fB10\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_arc_dnode_reduce_percent\fR (ulong)
.ad
.RS 12n
Percentage of ARC dnodes to try to scan in response to demand for non-metadata
when the number of bytes consumed by dnodes exceeds \fBzfs_arc_dnode_limit\fR.

.sp
Default value: \fB10\fR% of the number of dnodes in the ARC.
.RE

.sp
.ne 2
.na
\fBzfs_arc_average_blocksize\fR (int)
.ad
.RS 12n
The ARC's buffer hash table is sized based on the assumption of an average
block size of \fBzfs_arc_average_blocksize\fR (default 8K).  This works out
to roughly 1MB of hash table per 1GB of physical memory with 8-byte pointers.
For configurations with a known larger average block size this value can be
increased to reduce the memory footprint.

.sp
Default value: \fB8192\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_evict_batch_limit\fR (int)
.ad
.RS 12n
Number ARC headers to evict per sub-list before proceeding to another sub-list.
This batch-style operation prevents entire sub-lists from being evicted at once
but comes at a cost of additional unlocking and locking.
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_grow_retry\fR (int)
.ad
.RS 12n
If set to a non zero value, it will replace the arc_grow_retry value with this value.
The arc_grow_retry value (default 5) is the number of seconds the ARC will wait before
trying to resume growth after a memory pressure event.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_lotsfree_percent\fR (int)
.ad
.RS 12n
Throttle I/O when free system memory drops below this percentage of total
system memory.  Setting this value to 0 will disable the throttle.
.sp
Default value: \fB10\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_arc_max\fR (ulong)
.ad
.RS 12n
Max arc size of ARC in bytes. If set to 0 then it will consume 1/2 of system
RAM. This value must be at least 67108864 (64 megabytes).
.sp
This value can be changed dynamically with some caveats. It cannot be set back
to 0 while running and reducing it below the current ARC size will not cause
the ARC to shrink without memory pressure to induce shrinking.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_adjust_restarts\fR (ulong)
.ad
.RS 12n
The number of restart passes to make while scanning the ARC attempting
the free buffers in order to stay below the \fBzfs_arc_meta_limit\fR.
This value should not need to be tuned but is available to facilitate
performance analysis.
.sp
Default value: \fB4096\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_limit\fR (ulong)
.ad
.RS 12n
The maximum allowed size in bytes that meta data buffers are allowed to
consume in the ARC.  When this limit is reached meta data buffers will
be reclaimed even if the overall arc_c_max has not been reached.  This
value defaults to 0 which indicates that a percent which is based on
\fBzfs_arc_meta_limit_percent\fR of the ARC may be used for meta data.
.sp
This value my be changed dynamically except that it cannot be set back to 0
for a specific percent of the ARC; it must be set to an explicit value.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_limit_percent\fR (ulong)
.ad
.RS 12n
Percentage of ARC buffers that can be used for meta data.

See also \fBzfs_arc_meta_limit\fR which serves a similar purpose but has a
higher priority if set to nonzero value.

.sp
Default value: \fB75\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_min\fR (ulong)
.ad
.RS 12n
The minimum allowed size in bytes that meta data buffers may consume in
the ARC.  This value defaults to 0 which disables a floor on the amount
of the ARC devoted meta data.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_prune\fR (int)
.ad
.RS 12n
The number of dentries and inodes to be scanned looking for entries
which can be dropped.  This may be required when the ARC reaches the
\fBzfs_arc_meta_limit\fR because dentries and inodes can pin buffers
in the ARC.  Increasing this value will cause to dentry and inode caches
to be pruned more aggressively.  Setting this value to 0 will disable
pruning the inode and dentry caches.
.sp
Default value: \fB10,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_strategy\fR (int)
.ad
.RS 12n
Define the strategy for ARC meta data buffer eviction (meta reclaim strategy).
A value of 0 (META_ONLY) will evict only the ARC meta data buffers.
A value of 1 (BALANCED) indicates that additional data buffers may be evicted if
that is required to in order to evict the required number of meta data buffers.
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_min\fR (ulong)
.ad
.RS 12n
Min arc size of ARC in bytes. If set to 0 then arc_c_min will default to
consuming the larger of 32M or 1/32 of total system memory.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_min_prefetch_ms\fR (int)
.ad
.RS 12n
Minimum time prefetched blocks are locked in the ARC, specified in ms.
A value of \fB0\fR will default to 1000 ms.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_min_prescient_prefetch_ms\fR (int)
.ad
.RS 12n
Minimum time "prescient prefetched" blocks are locked in the ARC, specified
in ms. These blocks are meant to be prefetched fairly aggresively ahead of
the code that may use them. A value of \fB0\fR will default to 6000 ms.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_max_missing_tvds\fR (int)
.ad
.RS 12n
Number of missing top-level vdevs which will be allowed during
pool import (only in read-only mode).
.sp
Default value: \fB0\fR
.RE

.sp
.ne 2
.na
\fBzfs_multilist_num_sublists\fR (int)
.ad
.RS 12n
To allow more fine-grained locking, each ARC state contains a series
of lists for both data and meta data objects.  Locking is performed at
the level of these "sub-lists".  This parameters controls the number of
sub-lists per ARC state, and also applies to other uses of the
multilist data structure.
.sp
Default value: \fB4\fR or the number of online CPUs, whichever is greater
.RE

.sp
.ne 2
.na
\fBzfs_arc_overflow_shift\fR (int)
.ad
.RS 12n
The ARC size is considered to be overflowing if it exceeds the current
ARC target size (arc_c) by a threshold determined by this parameter.
The threshold is calculated as a fraction of arc_c using the formula
"arc_c >> \fBzfs_arc_overflow_shift\fR".

The default value of 8 causes the ARC to be considered to be overflowing
if it exceeds the target size by 1/256th (0.3%) of the target size.

When the ARC is overflowing, new buffer allocations are stalled until
the reclaim thread catches up and the overflow condition no longer exists.
.sp
Default value: \fB8\fR.
.RE

.sp
.ne 2
.na

\fBzfs_arc_p_min_shift\fR (int)
.ad
.RS 12n
If set to a non zero value, this will update arc_p_min_shift (default 4)
with the new value.
arc_p_min_shift is used to shift of arc_c for calculating both min and max
max arc_p
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_p_dampener_disable\fR (int)
.ad
.RS 12n
Disable arc_p adapt dampener
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE

.sp
.ne 2
.na
\fBzfs_arc_shrink_shift\fR (int)
.ad
.RS 12n
If set to a non zero value, this will update arc_shrink_shift (default 7)
with the new value.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_pc_percent\fR (uint)
.ad
.RS 12n
Percent of pagecache to reclaim arc to

This tunable allows ZFS arc to play more nicely with the kernel's LRU
pagecache. It can guarantee that the arc size won't collapse under scanning
pressure on the pagecache, yet still allows arc to be reclaimed down to
zfs_arc_min if necessary. This value is specified as percent of pagecache
size (as measured by NR_FILE_PAGES) where that percent may exceed 100. This
only operates during memory pressure/reclaim.
.sp
Default value: \fB0\fR% (disabled).
.RE

.sp
.ne 2
.na
\fBzfs_arc_sys_free\fR (ulong)
.ad
.RS 12n
The target number of bytes the ARC should leave as free memory on the system.
Defaults to the larger of 1/64 of physical memory or 512K.  Setting this
option to a non-zero value will override the default.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_autoimport_disable\fR (int)
.ad
.RS 12n
Disable pool import at module load by ignoring the cache file (typically \fB/etc/zfs/zpool.cache\fR).
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBzfs_checksums_per_second\fR (int)
.ad
.RS 12n
Rate limit checksum events to this many per second.  Note that this should
not be set below the zed thresholds (currently 10 checksums over 10 sec)
or else zed may not trigger any action.
.sp
Default value: 20
.RE

.sp
.ne 2
.na
\fBzfs_commit_timeout_pct\fR (int)
.ad
.RS 12n
This controls the amount of time that a ZIL block (lwb) will remain "open"
when it isn't "full", and it has a thread waiting for it to be committed to
stable storage.  The timeout is scaled based on a percentage of the last lwb
latency to avoid significantly impacting the latency of each individual
transaction record (itx).
.sp
Default value: \fB5\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_condense_indirect_vdevs_enable\fR (int)
.ad
.RS 12n
Enable condensing indirect vdev mappings.  When set to a non-zero value,
attempt to condense indirect vdev mappings if the mapping uses more than
\fBzfs_condense_min_mapping_bytes\fR bytes of memory and if the obsolete
space map object uses more than \fBzfs_condense_max_obsolete_bytes\fR
bytes on-disk.  The condensing process is an attempt to save memory by
removing obsolete mappings.
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_condense_max_obsolete_bytes\fR (ulong)
.ad
.RS 12n
Only attempt to condense indirect vdev mappings if the on-disk size
of the obsolete space map object is greater than this number of bytes
(see \fBfBzfs_condense_indirect_vdevs_enable\fR).
.sp
Default value: \fB1,073,741,824\fR.
.RE

.sp
.ne 2
.na
\fBzfs_condense_min_mapping_bytes\fR (ulong)
.ad
.RS 12n
Minimum size vdev mapping to attempt to condense (see
\fBzfs_condense_indirect_vdevs_enable\fR).
.sp
Default value: \fB131,072\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dbgmsg_enable\fR (int)
.ad
.RS 12n
Internally ZFS keeps a small log to facilitate debugging.  By default the log
is disabled, to enable it set this option to 1.  The contents of the log can
be accessed by reading the /proc/spl/kstat/zfs/dbgmsg file.  Writing 0 to
this proc file clears the log.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dbgmsg_maxsize\fR (int)
.ad
.RS 12n
The maximum size in bytes of the internal ZFS debug log.
.sp
Default value: \fB4M\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dbuf_state_index\fR (int)
.ad
.RS 12n
This feature is currently unused. It is normally used for controlling what
reporting is available under /proc/spl/kstat/zfs.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_deadman_enabled\fR (int)
.ad
.RS 12n
When a pool sync operation takes longer than \fBzfs_deadman_synctime_ms\fR
milliseconds, or when an individual I/O takes longer than
\fBzfs_deadman_ziotime_ms\fR milliseconds, then the operation is considered to
be "hung".  If \fBzfs_deadman_enabled\fR is set then the deadman behavior is
invoked as described by the \fBzfs_deadman_failmode\fR module option.
By default the deadman is enabled and configured to \fBwait\fR which results
in "hung" I/Os only being logged.  The deadman is automatically disabled
when a pool gets suspended.
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_deadman_failmode\fR (charp)
.ad
.RS 12n
Controls the failure behavior when the deadman detects a "hung" I/O.  Valid
values are \fBwait\fR, \fBcontinue\fR, and \fBpanic\fR.
.sp
\fBwait\fR - Wait for a "hung" I/O to complete.  For each "hung" I/O a
"deadman" event will be posted describing that I/O.
.sp
\fBcontinue\fR - Attempt to recover from a "hung" I/O by re-dispatching it
to the I/O pipeline if possible.
.sp
\fBpanic\fR - Panic the system.  This can be used to facilitate an automatic
fail-over to a properly configured fail-over partner.
.sp
Default value: \fBwait\fR.
.RE

.sp
.ne 2
.na
\fBzfs_deadman_checktime_ms\fR (int)
.ad
.RS 12n
Check time in milliseconds. This defines the frequency at which we check
for hung I/O and potentially invoke the \fBzfs_deadman_failmode\fR behavior.
.sp
Default value: \fB60,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_deadman_synctime_ms\fR (ulong)
.ad
.RS 12n
Interval in milliseconds after which the deadman is triggered and also
the interval after which a pool sync operation is considered to be "hung".
Once this limit is exceeded the deadman will be invoked every
\fBzfs_deadman_checktime_ms\fR milliseconds until the pool sync completes.
.sp
Default value: \fB600,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_deadman_ziotime_ms\fR (ulong)
.ad
.RS 12n
Interval in milliseconds after which the deadman is triggered and an
individual IO operation is considered to be "hung".  As long as the I/O
remains "hung" the deadman will be invoked every \fBzfs_deadman_checktime_ms\fR
milliseconds until the I/O completes.
.sp
Default value: \fB300,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dedup_prefetch\fR (int)
.ad
.RS 12n
Enable prefetching dedup-ed blks
.sp
Use \fB1\fR for yes and \fB0\fR to disable (default).
.RE

.sp
.ne 2
.na
\fBzfs_delay_min_dirty_percent\fR (int)
.ad
.RS 12n
Start to delay each transaction once there is this amount of dirty data,
expressed as a percentage of \fBzfs_dirty_data_max\fR.
This value should be >= zfs_vdev_async_write_active_max_dirty_percent.
See the section "ZFS TRANSACTION DELAY".
.sp
Default value: \fB60\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_delay_scale\fR (int)
.ad
.RS 12n
This controls how quickly the transaction delay approaches infinity.
Larger values cause longer delays for a given amount of dirty data.
.sp
For the smoothest delay, this value should be about 1 billion divided
by the maximum number of operations per second.  This will smoothly
handle between 10x and 1/10th this number.
.sp
See the section "ZFS TRANSACTION DELAY".
.sp
Note: \fBzfs_delay_scale\fR * \fBzfs_dirty_data_max\fR must be < 2^64.
.sp
Default value: \fB500,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_delays_per_second\fR (int)
.ad
.RS 12n
Rate limit IO delay events to this many per second.
.sp
Default value: 20
.RE

.sp
.ne 2
.na
\fBzfs_delete_blocks\fR (ulong)
.ad
.RS 12n
This is the used to define a large file for the purposes of delete.  Files
containing more than \fBzfs_delete_blocks\fR will be deleted asynchronously
while smaller files are deleted synchronously.  Decreasing this value will
reduce the time spent in an unlink(2) system call at the expense of a longer
delay before the freed space is available.
.sp
Default value: \fB20,480\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_max\fR (int)
.ad
.RS 12n
Determines the dirty space limit in bytes.  Once this limit is exceeded, new
writes are halted until space frees up. This parameter takes precedence
over \fBzfs_dirty_data_max_percent\fR.
See the section "ZFS TRANSACTION DELAY".
.sp
Default value: \fB10\fR% of physical RAM, capped at \fBzfs_dirty_data_max_max\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_max_max\fR (int)
.ad
.RS 12n
Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed in bytes.
This limit is only enforced at module load time, and will be ignored if
\fBzfs_dirty_data_max\fR is later changed.  This parameter takes
precedence over \fBzfs_dirty_data_max_max_percent\fR. See the section
"ZFS TRANSACTION DELAY".
.sp
Default value: \fB25\fR% of physical RAM.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_max_max_percent\fR (int)
.ad
.RS 12n
Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed as a
percentage of physical RAM.  This limit is only enforced at module load
time, and will be ignored if \fBzfs_dirty_data_max\fR is later changed.
The parameter \fBzfs_dirty_data_max_max\fR takes precedence over this
one. See the section "ZFS TRANSACTION DELAY".
.sp
Default value: \fB25\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_max_percent\fR (int)
.ad
.RS 12n
Determines the dirty space limit, expressed as a percentage of all
memory.  Once this limit is exceeded, new writes are halted until space frees
up.  The parameter \fBzfs_dirty_data_max\fR takes precedence over this
one.  See the section "ZFS TRANSACTION DELAY".
.sp
Default value: \fB10\fR%, subject to \fBzfs_dirty_data_max_max\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_sync\fR (int)
.ad
.RS 12n
Start syncing out a transaction group if there is at least this much dirty data.
.sp
Default value: \fB67,108,864\fR.
.RE

.sp
.ne 2
.na
\fBzfs_fletcher_4_impl\fR (string)
.ad
.RS 12n
Select a fletcher 4 implementation.
.sp
Supported selectors are: \fBfastest\fR, \fBscalar\fR, \fBsse2\fR, \fBssse3\fR,
\fBavx2\fR, \fBavx512f\fR, and \fBaarch64_neon\fR.
All of the selectors except \fBfastest\fR and \fBscalar\fR require instruction
set extensions to be available and will only appear if ZFS detects that they are
present at runtime. If multiple implementations of fletcher 4 are available,
the \fBfastest\fR will be chosen using a micro benchmark. Selecting \fBscalar\fR
results in the original, CPU based calculation, being used. Selecting any option
other than \fBfastest\fR and \fBscalar\fR results in vector instructions from
the respective CPU instruction set being used.
.sp
Default value: \fBfastest\fR.
.RE

.sp
.ne 2
.na
\fBzfs_free_bpobj_enabled\fR (int)
.ad
.RS 12n
Enable/disable the processing of the free_bpobj object.
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_async_block_max_blocks\fR (ulong)
.ad
.RS 12n
Maximum number of blocks freed in a single txg.
.sp
Default value: \fB100,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_override_estimate_recordsize\fR (ulong)
.ad
.RS 12n
Record size calculation override for zfs send estimates.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_read_max_active\fR (int)
.ad
.RS 12n
Maximum asynchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB3\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_read_min_active\fR (int)
.ad
.RS 12n
Minimum asynchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_write_active_max_dirty_percent\fR (int)
.ad
.RS 12n
When the pool has more than
\fBzfs_vdev_async_write_active_max_dirty_percent\fR dirty data, use
\fBzfs_vdev_async_write_max_active\fR to limit active async writes.  If
the dirty data is between min and max, the active I/O limit is linearly
interpolated. See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB60\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_write_active_min_dirty_percent\fR (int)
.ad
.RS 12n
When the pool has less than
\fBzfs_vdev_async_write_active_min_dirty_percent\fR dirty data, use
\fBzfs_vdev_async_write_min_active\fR to limit active async writes.  If
the dirty data is between min and max, the active I/O limit is linearly
interpolated. See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB30\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_write_max_active\fR (int)
.ad
.RS 12n
Maximum asynchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_write_min_active\fR (int)
.ad
.RS 12n
Minimum asynchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Lower values are associated with better latency on rotational media but poorer
resilver performance. The default value of 2 was chosen as a compromise. A
value of 3 has been shown to improve resilver performance further at a cost of
further increasing latency.
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_max_active\fR (int)
.ad
.RS 12n
The maximum number of I/Os active to each device.  Ideally, this will be >=
the sum of each queue's max_active.  It must be at least the sum of each
queue's min_active.  See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_scrub_max_active\fR (int)
.ad
.RS 12n
Maximum scrub I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_scrub_min_active\fR (int)
.ad
.RS 12n
Minimum scrub I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_sync_read_max_active\fR (int)
.ad
.RS 12n
Maximum synchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_sync_read_min_active\fR (int)
.ad
.RS 12n
Minimum synchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_sync_write_max_active\fR (int)
.ad
.RS 12n
Maximum synchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_sync_write_min_active\fR (int)
.ad
.RS 12n
Minimum synchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_queue_depth_pct\fR (int)
.ad
.RS 12n
Maximum number of queued allocations per top-level vdev expressed as
a percentage of \fBzfs_vdev_async_write_max_active\fR which allows the
system to detect devices that are more capable of handling allocations
and to allocate more blocks to those devices.  It allows for dynamic
allocation distribution when devices are imbalanced as fuller devices
will tend to be slower than empty devices.

See also \fBzio_dva_throttle_enabled\fR.
.sp
Default value: \fB1000\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_expire_snapshot\fR (int)
.ad
.RS 12n
Seconds to expire .zfs/snapshot
.sp
Default value: \fB300\fR.
.RE

.sp
.ne 2
.na
\fBzfs_admin_snapshot\fR (int)
.ad
.RS 12n
Allow the creation, removal, or renaming of entries in the .zfs/snapshot
directory to cause the creation, destruction, or renaming of snapshots.
When enabled this functionality works both locally and over NFS exports
which have the 'no_root_squash' option set. This functionality is disabled
by default.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_flags\fR (int)
.ad
.RS 12n
Set additional debugging flags. The following flags may be bitwise-or'd
together.
.sp
.TS
box;
rB lB
lB lB
r l.
Value	Symbolic Name
	Description
_
1	ZFS_DEBUG_DPRINTF
	Enable dprintf entries in the debug log.
_
2	ZFS_DEBUG_DBUF_VERIFY *
	Enable extra dbuf verifications.
_
4	ZFS_DEBUG_DNODE_VERIFY *
	Enable extra dnode verifications.
_
8	ZFS_DEBUG_SNAPNAMES
	Enable snapshot name verification.
_
16	ZFS_DEBUG_MODIFY
	Check for illegally modified ARC buffers.
_
64	ZFS_DEBUG_ZIO_FREE
	Enable verification of block frees.
_
128	ZFS_DEBUG_HISTOGRAM_VERIFY
	Enable extra spacemap histogram verifications.
_
256	ZFS_DEBUG_METASLAB_VERIFY
	Verify space accounting on disk matches in-core range_trees.
_
512	ZFS_DEBUG_SET_ERROR
	Enable SET_ERROR and dprintf entries in the debug log.
.TE
.sp
* Requires debug build.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_free_leak_on_eio\fR (int)
.ad
.RS 12n
If destroy encounters an EIO while reading metadata (e.g. indirect
blocks), space referenced by the missing metadata can not be freed.
Normally this causes the background destroy to become "stalled", as
it is unable to make forward progress.  While in this stalled state,
all remaining space to free from the error-encountering filesystem is
"temporarily leaked".  Set this flag to cause it to ignore the EIO,
permanently leak the space from indirect blocks that can not be read,
and continue to free everything else that it can.

The default, "stalling" behavior is useful if the storage partially
fails (i.e. some but not all i/os fail), and then later recovers.  In
this case, we will be able to continue pool operations while it is
partially failed, and when it recovers, we can continue to free the
space, with no leaks.  However, note that this case is actually
fairly rare.

Typically pools either (a) fail completely (but perhaps temporarily,
e.g. a top-level vdev going offline), or (b) have localized,
permanent errors (e.g. disk returns the wrong data due to bit flip or
firmware bug).  In case (a), this setting does not matter because the
pool will be suspended and the sync thread will not be able to make
forward progress regardless.  In case (b), because the error is
permanent, the best we can do is leak the minimum amount of space,
which is what setting this flag will do.  Therefore, it is reasonable
for this flag to normally be set, but we chose the more conservative
approach of not setting it, so that there is no possibility of
leaking space in the "partial temporary" failure case.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_free_min_time_ms\fR (int)
.ad
.RS 12n
During a \fBzfs destroy\fR operation using \fBfeature@async_destroy\fR a minimum
of this much time will be spent working on freeing blocks per txg.
.sp
Default value: \fB1,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_immediate_write_sz\fR (long)
.ad
.RS 12n
Largest data block to write to zil. Larger blocks will be treated as if the
dataset being written to had the property setting \fBlogbias=throughput\fR.
.sp
Default value: \fB32,768\fR.
.RE

.sp
.ne 2
.na
\fBzfs_lua_max_instrlimit\fR (ulong)
.ad
.RS 12n
The maximum execution time limit that can be set for a ZFS channel program,
specified as a number of Lua instructions.
.sp
Default value: \fB100,000,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_lua_max_memlimit\fR (ulong)
.ad
.RS 12n
The maximum memory limit that can be set for a ZFS channel program, specified
in bytes.
.sp
Default value: \fB104,857,600\fR.
.RE

.sp
.ne 2
.na
\fBzfs_max_dataset_nesting\fR (int)
.ad
.RS 12n
The maximum depth of nested datasets.  This value can be tuned temporarily to
fix existing datasets that exceed the predefined limit.
.sp
Default value: \fB50\fR.
.RE

.sp
.ne 2
.na
\fBzfs_max_recordsize\fR (int)
.ad
.RS 12n
We currently support block sizes from 512 bytes to 16MB.  The benefits of
larger blocks, and thus larger IO, need to be weighed against the cost of
COWing a giant block to modify one byte.  Additionally, very large blocks
can have an impact on i/o latency, and also potentially on the memory
allocator.  Therefore, we do not allow the recordsize to be set larger than
zfs_max_recordsize (default 1MB).  Larger blocks can be created by changing
this tunable, and pools with larger blocks can always be imported and used,
regardless of this setting.
.sp
Default value: \fB1,048,576\fR.
.RE

.sp
.ne 2
.na
\fBzfs_metaslab_fragmentation_threshold\fR (int)
.ad
.RS 12n
Allow metaslabs to keep their active state as long as their fragmentation
percentage is less than or equal to this value. An active metaslab that
exceeds this threshold will no longer keep its active status allowing
better metaslabs to be selected.
.sp
Default value: \fB70\fR.
.RE

.sp
.ne 2
.na
\fBzfs_mg_fragmentation_threshold\fR (int)
.ad
.RS 12n
Metaslab groups are considered eligible for allocations if their
fragmentation metric (measured as a percentage) is less than or equal to
this value. If a metaslab group exceeds this threshold then it will be
skipped unless all metaslab groups within the metaslab class have also
crossed this threshold.
.sp
Default value: \fB85\fR.
.RE

.sp
.ne 2
.na
\fBzfs_mg_noalloc_threshold\fR (int)
.ad
.RS 12n
Defines a threshold at which metaslab groups should be eligible for
allocations.  The value is expressed as a percentage of free space
beyond which a metaslab group is always eligible for allocations.
If a metaslab group's free space is less than or equal to the
threshold, the allocator will avoid allocating to that group
unless all groups in the pool have reached the threshold.  Once all
groups have reached the threshold, all groups are allowed to accept
allocations.  The default value of 0 disables the feature and causes
all metaslab groups to be eligible for allocations.

This parameter allows one to deal with pools having heavily imbalanced
vdevs such as would be the case when a new vdev has been added.
Setting the threshold to a non-zero percentage will stop allocations
from being made to vdevs that aren't filled to the specified percentage
and allow lesser filled vdevs to acquire more allocations than they
otherwise would under the old \fBzfs_mg_alloc_failures\fR facility.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_multihost_history\fR (int)
.ad
.RS 12n
Historical statistics for the last N multihost updates will be available in
\fB/proc/spl/kstat/zfs/<pool>/multihost\fR
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_multihost_interval\fR (ulong)
.ad
.RS 12n
Used to control the frequency of multihost writes which are performed when the
\fBmultihost\fR pool property is on.  This is one factor used to determine
the length of the activity check during import.
.sp
The multihost write period is \fBzfs_multihost_interval / leaf-vdevs\fR milliseconds.
This means that on average a multihost write will be issued for each leaf vdev every
\fBzfs_multihost_interval\fR milliseconds.  In practice, the observed period can
vary with the I/O load and this observed value is the delay which is stored in
the uberblock.
.sp
On import the activity check waits a minimum amount of time determined by
\fBzfs_multihost_interval * zfs_multihost_import_intervals\fR.  The activity
check time may be further extended if the value of mmp delay found in the best
uberblock indicates actual multihost updates happened at longer intervals than
\fBzfs_multihost_interval\fR.  A minimum value of \fB100ms\fR is enforced.
.sp
Default value: \fB1000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_multihost_import_intervals\fR (uint)
.ad
.RS 12n
Used to control the duration of the activity test on import.  Smaller values of
\fBzfs_multihost_import_intervals\fR will reduce the import time but increase
the risk of failing to detect an active pool.  The total activity check time is
never allowed to drop below one second.  A value of 0 is ignored and treated as
if it was set to 1
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_multihost_fail_intervals\fR (uint)
.ad
.RS 12n
Controls the behavior of the pool when multihost write failures are detected.
.sp
When \fBzfs_multihost_fail_intervals = 0\fR then multihost write failures are ignored.
The failures will still be reported to the ZED which depending on its
configuration may take action such as suspending the pool or offlining a device.
.sp
When \fBzfs_multihost_fail_intervals > 0\fR then sequential multihost write failures
will cause the pool to be suspended.  This occurs when
\fBzfs_multihost_fail_intervals * zfs_multihost_interval\fR milliseconds have
passed since the last successful multihost write.  This guarantees the activity test
will see multihost writes if the pool is imported.
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBzfs_no_scrub_io\fR (int)
.ad
.RS 12n
Set for no scrub I/O. This results in scrubs not actually scrubbing data and
simply doing a metadata crawl of the pool instead.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_no_scrub_prefetch\fR (int)
.ad
.RS 12n
Set to disable block prefetching for scrubs.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_nocacheflush\fR (int)
.ad
.RS 12n
Disable cache flush operations on disks when writing. Beware, this may cause
corruption if disks re-order writes.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_nopwrite_enabled\fR (int)
.ad
.RS 12n
Enable NOP writes
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE

.sp
.ne 2
.na
\fBzfs_dmu_offset_next_sync\fR (int)
.ad
.RS 12n
Enable forcing txg sync to find holes. When enabled forces ZFS to act
like prior versions when SEEK_HOLE or SEEK_DATA flags are used, which
when a dnode is dirty causes txg's to be synced so that this data can be
found.
.sp
Use \fB1\fR for yes and \fB0\fR to disable (default).
.RE

.sp
.ne 2
.na
\fBzfs_pd_bytes_max\fR (int)
.ad
.RS 12n
The number of bytes which should be prefetched during a pool traversal
(eg: \fBzfs send\fR or other data crawling operations)
.sp
Default value: \fB52,428,800\fR.
.RE

.sp
.ne 2
.na
\fBzfs_per_txg_dirty_frees_percent \fR (ulong)
.ad
.RS 12n
Tunable to control percentage of dirtied blocks from frees in one TXG.
After this threshold is crossed, additional dirty blocks from frees
wait until the next TXG.
A value of zero will disable this throttle.
.sp
Default value: \fB30\fR and \fB0\fR to disable.
.RE



.sp
.ne 2
.na
\fBzfs_prefetch_disable\fR (int)
.ad
.RS 12n
This tunable disables predictive prefetch.  Note that it leaves "prescient"
prefetch (e.g. prefetch for zfs send) intact.  Unlike predictive prefetch,
prescient prefetch never issues i/os that end up not being needed, so it
can't hurt performance.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_read_chunk_size\fR (long)
.ad
.RS 12n
Bytes to read per chunk
.sp
Default value: \fB1,048,576\fR.
.RE

.sp
.ne 2
.na
\fBzfs_read_history\fR (int)
.ad
.RS 12n
Historical statistics for the last N reads will be available in
\fB/proc/spl/kstat/zfs/<pool>/reads\fR
.sp
Default value: \fB0\fR (no data is kept).
.RE

.sp
.ne 2
.na
\fBzfs_read_history_hits\fR (int)
.ad
.RS 12n
Include cache hits in read history
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_reconstruct_indirect_combinations_max\fR (int)
.ad
.RS 12na
If an indirect split block contains more than this many possible unique
combinations when being reconstructed, consider it too computationally
expensive to check them all. Instead, try at most
\fBzfs_reconstruct_indirect_combinations_max\fR randomly-selected
combinations each time the block is accessed.  This allows all segment
copies to participate fairly in the reconstruction when all combinations
cannot be checked and prevents repeated use of one bad copy.
.sp
Default value: \fB100\fR.
.RE

.sp
.ne 2
.na
\fBzfs_recover\fR (int)
.ad
.RS 12n
Set to attempt to recover from fatal errors. This should only be used as a
last resort, as it typically results in leaked space, or worse.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_resilver_min_time_ms\fR (int)
.ad
.RS 12n
Resilvers are processed by the sync thread. While resilvering it will spend
at least this much time working on a resilver between txg flushes.
.sp
Default value: \fB3,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scan_ignore_errors\fR (int)
.ad
.RS 12n
If set to a nonzero value, remove the DTL (dirty time list) upon
completion of a pool scan (scrub) even if there were unrepairable
errors.  It is intended to be used during pool repair or recovery to
stop resilvering when the pool is next imported.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scrub_min_time_ms\fR (int)
.ad
.RS 12n
Scrubs are processed by the sync thread. While scrubbing it will spend
at least this much time working on a scrub between txg flushes.
.sp
Default value: \fB1,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scan_checkpoint_intval\fR (int)
.ad
.RS 12n
To preserve progress across reboots the sequential scan algorithm periodically
needs to stop metadata scanning and issue all the verifications I/Os to disk.
The frequency of this flushing is determined by the
\fBzfs_scan_checkpoint_intval\fR tunable.
.sp
Default value: \fB7200\fR seconds (every 2 hours).
.RE

.sp
.ne 2
.na
\fBzfs_scan_fill_weight\fR (int)
.ad
.RS 12n
This tunable affects how scrub and resilver I/O segments are ordered. A higher
number indicates that we care more about how filled in a segment is, while a
lower number indicates we care more about the size of the extent without
considering the gaps within a segment. This value is only tunable upon module
insertion. Changing the value afterwards will have no affect on scrub or
resilver performance.
.sp
Default value: \fB3\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scan_issue_strategy\fR (int)
.ad
.RS 12n
Determines the order that data will be verified while scrubbing or resilvering.
If set to \fB1\fR, data will be verified as sequentially as possible, given the
amount of memory reserved for scrubbing (see \fBzfs_scan_mem_lim_fact\fR). This
may improve scrub performance if the pool's data is very fragmented. If set to
\fB2\fR, the largest mostly-contiguous chunk of found data will be verified
first. By deferring scrubbing of small segments, we may later find adjacent data
to coalesce and increase the segment size. If set to \fB0\fR, zfs will use
strategy \fB1\fR during normal verification and strategy \fB2\fR while taking a
checkpoint.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scan_legacy\fR (int)
.ad
.RS 12n
A value of 0 indicates that scrubs and resilvers will gather metadata in
memory before issuing sequential I/O. A value of 1 indicates that the legacy
algorithm will be used where I/O is initiated as soon as it is discovered.
Changing this value to 0 will not affect scrubs or resilvers that are already
in progress.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scan_max_ext_gap\fR (int)
.ad
.RS 12n
Indicates the largest gap in bytes between scrub / resilver I/Os that will still
be considered sequential for sorting purposes. Changing this value will not
affect scrubs or resilvers that are already in progress.
.sp
Default value: \fB2097152 (2 MB)\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scan_mem_lim_fact\fR (int)
.ad
.RS 12n
Maximum fraction of RAM used for I/O sorting by sequential scan algorithm.
This tunable determines the hard limit for I/O sorting memory usage.
When the hard limit is reached we stop scanning metadata and start issuing
data verification I/O. This is done until we get below the soft limit.
.sp
Default value: \fB20\fR which is 5% of RAM (1/20).
.RE

.sp
.ne 2
.na
\fBzfs_scan_mem_lim_soft_fact\fR (int)
.ad
.RS 12n
The fraction of the hard limit used to determined the soft limit for I/O sorting
by the sequential scan algorithm. When we cross this limit from bellow no action
is taken. When we cross this limit from above it is because we are issuing
verification I/O. In this case (unless the metadata scan is done) we stop
issuing verification I/O and start scanning metadata again until we get to the
hard limit.
.sp
Default value: \fB20\fR which is 5% of the hard limit (1/20).
.RE

.sp
.ne 2
.na
\fBzfs_scan_vdev_limit\fR (int)
.ad
.RS 12n
Maximum amount of data that can be concurrently issued at once for scrubs and
resilvers per leaf device, given in bytes.
.sp
Default value: \fB41943040\fR.
.RE

.sp
.ne 2
.na
\fBzfs_send_corrupt_data\fR (int)
.ad
.RS 12n
Allow sending of corrupt data (ignore read/checksum errors when sending data)
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_send_queue_length\fR (int)
.ad
.RS 12n
The maximum number of bytes allowed in the \fBzfs send\fR queue. This value
must be at least twice the maximum block size in use.
.sp
Default value: \fB16,777,216\fR.
.RE

.sp
.ne 2
.na
\fBzfs_recv_queue_length\fR (int)
.ad
.RS 12n
.sp
The maximum number of bytes allowed in the \fBzfs receive\fR queue. This value
must be at least twice the maximum block size in use.
.sp
Default value: \fB16,777,216\fR.
.RE

.sp
.ne 2
.na
\fBzfs_sync_pass_deferred_free\fR (int)
.ad
.RS 12n
Flushing of data to disk is done in passes. Defer frees starting in this pass
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_spa_discard_memory_limit\fR (int)
.ad
.RS 12n
Maximum memory used for prefetching a checkpoint's space map on each
vdev while discarding the checkpoint.
.sp
Default value: \fB16,777,216\fR.
.RE

.sp
.ne 2
.na
\fBzfs_sync_pass_dont_compress\fR (int)
.ad
.RS 12n
Don't compress starting in this pass
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBzfs_sync_pass_rewrite\fR (int)
.ad
.RS 12n
Rewrite new block pointers starting in this pass
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_sync_taskq_batch_pct\fR (int)
.ad
.RS 12n
This controls the number of threads used by the dp_sync_taskq.  The default
value of 75% will create a maximum of one thread per cpu.
.sp
Default value: \fB75\fR%.
.RE

.sp
.ne 2
.na
\fBzfs_txg_history\fR (int)
.ad
.RS 12n
Historical statistics for the last N txgs will be available in
\fB/proc/spl/kstat/zfs/<pool>/txgs\fR
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_txg_timeout\fR (int)
.ad
.RS 12n
Flush dirty data to disk at least every N seconds (maximum txg duration)
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_aggregation_limit\fR (int)
.ad
.RS 12n
Max vdev I/O aggregation size
.sp
Default value: \fB131,072\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_cache_bshift\fR (int)
.ad
.RS 12n
Shift size to inflate reads too
.sp
Default value: \fB16\fR (effectively 65536).
.RE

.sp
.ne 2
.na
\fBzfs_vdev_cache_max\fR (int)
.ad
.RS 12n
Inflate reads smaller than this value to meet the \fBzfs_vdev_cache_bshift\fR
size (default 64k).
.sp
Default value: \fB16384\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_cache_size\fR (int)
.ad
.RS 12n
Total size of the per-disk cache in bytes.
.sp
Currently this feature is disabled as it has been found to not be helpful
for performance and in some cases harmful.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_mirror_rotating_inc\fR (int)
.ad
.RS 12n
A number by which the balancing algorithm increments the load calculation for
the purpose of selecting the least busy mirror member when an I/O immediately
follows its predecessor on rotational vdevs for the purpose of making decisions
based on load.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_mirror_rotating_seek_inc\fR (int)
.ad
.RS 12n
A number by which the balancing algorithm increments the load calculation for
the purpose of selecting the least busy mirror member when an I/O lacks
locality as defined by the zfs_vdev_mirror_rotating_seek_offset.  I/Os within
this that are not immediately following the previous I/O are incremented by
half.
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_mirror_rotating_seek_offset\fR (int)
.ad
.RS 12n
The maximum distance for the last queued I/O in which the balancing algorithm
considers an I/O to have locality.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1048576\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_mirror_non_rotating_inc\fR (int)
.ad
.RS 12n
A number by which the balancing algorithm increments the load calculation for
the purpose of selecting the least busy mirror member on non-rotational vdevs
when I/Os do not immediately follow one another.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_mirror_non_rotating_seek_inc\fR (int)
.ad
.RS 12n
A number by which the balancing algorithm increments the load calculation for
the purpose of selecting the least busy mirror member when an I/O lacks
locality as defined by the zfs_vdev_mirror_rotating_seek_offset. I/Os within
this that are not immediately following the previous I/O are incremented by
half.
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_read_gap_limit\fR (int)
.ad
.RS 12n
Aggregate read I/O operations if the gap on-disk between them is within this
threshold.
.sp
Default value: \fB32,768\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_scheduler\fR (charp)
.ad
.RS 12n
Set the Linux I/O scheduler on whole disk vdevs to this scheduler. Valid options
are noop, cfq, bfq & deadline
.sp
Default value: \fBnoop\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_write_gap_limit\fR (int)
.ad
.RS 12n
Aggregate write I/O over gap
.sp
Default value: \fB4,096\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_raidz_impl\fR (string)
.ad
.RS 12n
Parameter for selecting raidz parity implementation to use.

Options marked (always) below may be selected on module load as they are
supported on all systems.
The remaining options may only be set after the module is loaded, as they
are available only if the implementations are compiled in and supported
on the running system.

Once the module is loaded, the content of
/sys/module/zfs/parameters/zfs_vdev_raidz_impl will show available options
with the currently selected one enclosed in [].
Possible options are:
  fastest  - (always) implementation selected using built-in benchmark
  original - (always) original raidz implementation
  scalar   - (always) scalar raidz implementation
  sse2     - implementation using SSE2 instruction set (64bit x86 only)
  ssse3    - implementation using SSSE3 instruction set (64bit x86 only)
  avx2     - implementation using AVX2 instruction set (64bit x86 only)
  avx512f  - implementation using AVX512F instruction set (64bit x86 only)
  avx512bw - implementation using AVX512F & AVX512BW instruction sets (64bit x86 only)
  aarch64_neon - implementation using NEON (Aarch64/64 bit ARMv8 only)
  aarch64_neonx2 - implementation using NEON with more unrolling (Aarch64/64 bit ARMv8 only)
.sp
Default value: \fBfastest\fR.
.RE

.sp
.ne 2
.na
\fBzfs_zevent_cols\fR (int)
.ad
.RS 12n
When zevents are logged to the console use this as the word wrap width.
.sp
Default value: \fB80\fR.
.RE

.sp
.ne 2
.na
\fBzfs_zevent_console\fR (int)
.ad
.RS 12n
Log events to the console
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_zevent_len_max\fR (int)
.ad
.RS 12n
Max event queue length. A value of 0 will result in a calculated value which
increases with the number of CPUs in the system (minimum 64 events). Events
in the queue can be viewed with the \fBzpool events\fR command.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_zil_clean_taskq_maxalloc\fR (int)
.ad
.RS 12n
The maximum number of taskq entries that are allowed to be cached.  When this
limit is exceeded transaction records (itxs) will be cleaned synchronously.
.sp
Default value: \fB1048576\fR.
.RE

.sp
.ne 2
.na
\fBzfs_zil_clean_taskq_minalloc\fR (int)
.ad
.RS 12n
The number of taskq entries that are pre-populated when the taskq is first
created and are immediately available for use.
.sp
Default value: \fB1024\fR.
.RE

.sp
.ne 2
.na
\fBzfs_zil_clean_taskq_nthr_pct\fR (int)
.ad
.RS 12n
This controls the number of threads used by the dp_zil_clean_taskq.  The default
value of 100% will create a maximum of one thread per cpu.
.sp
Default value: \fB100\fR%.
.RE

.sp
.ne 2
.na
\fBzil_replay_disable\fR (int)
.ad
.RS 12n
Disable intent logging replay. Can be disabled for recovery from corrupted
ZIL
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzil_slog_bulk\fR (ulong)
.ad
.RS 12n
Limit SLOG write size per commit executed with synchronous priority.
Any writes above that will be executed with lower (asynchronous) priority
to limit potential SLOG device abuse by single active ZIL writer.
.sp
Default value: \fB786,432\fR.
.RE

.sp
.ne 2
.na
\fBzio_decompress_fail_fraction\fR (int)
.ad
.RS 12n
If non-zero, this value represents the denominator of the probability that zfs
should induce a decompression failure. For instance, for a 5% decompression
failure rate, this value should be set to 20.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzio_delay_max\fR (int)
.ad
.RS 12n
A zevent will be logged if a ZIO operation takes more than N milliseconds to
complete. Note that this is only a logging facility, not a timeout on
operations.
.sp
Default value: \fB30,000\fR.
.RE

.sp
.ne 2
.na
\fBzio_dva_throttle_enabled\fR (int)
.ad
.RS 12n
Throttle block allocations in the ZIO pipeline. This allows for
dynamic allocation distribution when devices are imbalanced.
When enabled, the maximum number of pending allocations per top-level vdev
is limited by \fBzfs_vdev_queue_depth_pct\fR.
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzio_requeue_io_start_cut_in_line\fR (int)
.ad
.RS 12n
Prioritize requeued I/O
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzio_taskq_batch_pct\fR (uint)
.ad
.RS 12n
Percentage of online CPUs (or CPU cores, etc) which will run a worker thread
for IO. These workers are responsible for IO work such as compression and
checksum calculations. Fractional number of CPUs will be rounded down.
.sp
The default value of 75 was chosen to avoid using all CPUs which can result in
latency issues and inconsistent application performance, especially when high
compression is enabled.
.sp
Default value: \fB75\fR.
.RE

.sp
.ne 2
.na
\fBzvol_inhibit_dev\fR (uint)
.ad
.RS 12n
Do not create zvol device nodes. This may slightly improve startup time on
systems with a very large number of zvols.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzvol_major\fR (uint)
.ad
.RS 12n
Major number for zvol block devices
.sp
Default value: \fB230\fR.
.RE

.sp
.ne 2
.na
\fBzvol_max_discard_blocks\fR (ulong)
.ad
.RS 12n
Discard (aka TRIM) operations done on zvols will be done in batches of this
many blocks, where block size is determined by the \fBvolblocksize\fR property
of a zvol.
.sp
Default value: \fB16,384\fR.
.RE

.sp
.ne 2
.na
\fBzvol_prefetch_bytes\fR (uint)
.ad
.RS 12n
When adding a zvol to the system prefetch \fBzvol_prefetch_bytes\fR
from the start and end of the volume.  Prefetching these regions
of the volume is desirable because they are likely to be accessed
immediately by \fBblkid(8)\fR or by the kernel scanning for a partition
table.
.sp
Default value: \fB131,072\fR.
.RE

.sp
.ne 2
.na
\fBzvol_request_sync\fR (uint)
.ad
.RS 12n
When processing I/O requests for a zvol submit them synchronously.  This
effectively limits the queue depth to 1 for each I/O submitter.  When set
to 0 requests are handled asynchronously by a thread pool.  The number of
requests which can be handled concurrently is controller by \fBzvol_threads\fR.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzvol_threads\fR (uint)
.ad
.RS 12n
Max number of threads which can handle zvol I/O requests concurrently.
.sp
Default value: \fB32\fR.
.RE

.sp
.ne 2
.na
\fBzvol_volmode\fR (uint)
.ad
.RS 12n
Defines zvol block devices behaviour when \fBvolmode\fR is set to \fBdefault\fR.
Valid values are \fB1\fR (full), \fB2\fR (dev) and \fB3\fR (none).
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_qat_disable\fR (int)
.ad
.RS 12n
This tunable disables qat hardware acceleration for gzip compression and.
AES-GCM encryption. It is available only if qat acceleration is compiled in
and the qat driver is present.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.SH ZFS I/O SCHEDULER
ZFS issues I/O operations to leaf vdevs to satisfy and complete I/Os.
The I/O scheduler determines when and in what order those operations are
issued.  The I/O scheduler divides operations into five I/O classes
prioritized in the following order: sync read, sync write, async read,
async write, and scrub/resilver.  Each queue defines the minimum and
maximum number of concurrent operations that may be issued to the
device.  In addition, the device has an aggregate maximum,
\fBzfs_vdev_max_active\fR. Note that the sum of the per-queue minimums
must not exceed the aggregate maximum.  If the sum of the per-queue
maximums exceeds the aggregate maximum, then the number of active I/Os
may reach \fBzfs_vdev_max_active\fR, in which case no further I/Os will
be issued regardless of whether all per-queue minimums have been met.
.sp
For many physical devices, throughput increases with the number of
concurrent operations, but latency typically suffers. Further, physical
devices typically have a limit at which more concurrent operations have no
effect on throughput or can actually cause it to decrease.
.sp
The scheduler selects the next operation to issue by first looking for an
I/O class whose minimum has not been satisfied. Once all are satisfied and
the aggregate maximum has not been hit, the scheduler looks for classes
whose maximum has not been satisfied. Iteration through the I/O classes is
done in the order specified above. No further operations are issued if the
aggregate maximum number of concurrent operations has been hit or if there
are no operations queued for an I/O class that has not hit its maximum.
Every time an I/O is queued or an operation completes, the I/O scheduler
looks for new operations to issue.
.sp
In general, smaller max_active's will lead to lower latency of synchronous
operations.  Larger max_active's may lead to higher overall throughput,
depending on underlying storage.
.sp
The ratio of the queues' max_actives determines the balance of performance
between reads, writes, and scrubs.  E.g., increasing
\fBzfs_vdev_scrub_max_active\fR will cause the scrub or resilver to complete
more quickly, but reads and writes to have higher latency and lower throughput.
.sp
All I/O classes have a fixed maximum number of outstanding operations
except for the async write class. Asynchronous writes represent the data
that is committed to stable storage during the syncing stage for
transaction groups. Transaction groups enter the syncing state
periodically so the number of queued async writes will quickly burst up
and then bleed down to zero. Rather than servicing them as quickly as
possible, the I/O scheduler changes the maximum number of active async
write I/Os according to the amount of dirty data in the pool.  Since
both throughput and latency typically increase with the number of
concurrent operations issued to physical devices, reducing the
burstiness in the number of concurrent operations also stabilizes the
response time of operations from other -- and in particular synchronous
-- queues. In broad strokes, the I/O scheduler will issue more
concurrent operations from the async write queue as there's more dirty
data in the pool.
.sp
Async Writes
.sp
The number of concurrent operations issued for the async write I/O class
follows a piece-wise linear function defined by a few adjustable points.
.nf

       |              o---------| <-- zfs_vdev_async_write_max_active
  ^    |             /^         |
  |    |            / |         |
active |           /  |         |
 I/O   |          /   |         |
count  |         /    |         |
       |        /     |         |
       |-------o      |         | <-- zfs_vdev_async_write_min_active
      0|_______^______|_________|
       0%      |      |       100% of zfs_dirty_data_max
               |      |
               |      `-- zfs_vdev_async_write_active_max_dirty_percent
               `--------- zfs_vdev_async_write_active_min_dirty_percent

.fi
Until the amount of dirty data exceeds a minimum percentage of the dirty
data allowed in the pool, the I/O scheduler will limit the number of
concurrent operations to the minimum. As that threshold is crossed, the
number of concurrent operations issued increases linearly to the maximum at
the specified maximum percentage of the dirty data allowed in the pool.
.sp
Ideally, the amount of dirty data on a busy pool will stay in the sloped
part of the function between \fBzfs_vdev_async_write_active_min_dirty_percent\fR
and \fBzfs_vdev_async_write_active_max_dirty_percent\fR. If it exceeds the
maximum percentage, this indicates that the rate of incoming data is
greater than the rate that the backend storage can handle. In this case, we
must further throttle incoming writes, as described in the next section.

.SH ZFS TRANSACTION DELAY
We delay transactions when we've determined that the backend storage
isn't able to accommodate the rate of incoming writes.
.sp
If there is already a transaction waiting, we delay relative to when
that transaction will finish waiting.  This way the calculated delay time
is independent of the number of threads concurrently executing
transactions.
.sp
If we are the only waiter, wait relative to when the transaction
started, rather than the current time.  This credits the transaction for
"time already served", e.g. reading indirect blocks.
.sp
The minimum time for a transaction to take is calculated as:
.nf
    min_time = zfs_delay_scale * (dirty - min) / (max - dirty)
    min_time is then capped at 100 milliseconds.
.fi
.sp
The delay has two degrees of freedom that can be adjusted via tunables.  The
percentage of dirty data at which we start to delay is defined by
\fBzfs_delay_min_dirty_percent\fR. This should typically be at or above
\fBzfs_vdev_async_write_active_max_dirty_percent\fR so that we only start to
delay after writing at full speed has failed to keep up with the incoming write
rate. The scale of the curve is defined by \fBzfs_delay_scale\fR. Roughly speaking,
this variable determines the amount of delay at the midpoint of the curve.
.sp
.nf
delay
 10ms +-------------------------------------------------------------*+
      |                                                             *|
  9ms +                                                             *+
      |                                                             *|
  8ms +                                                             *+
      |                                                            * |
  7ms +                                                            * +
      |                                                            * |
  6ms +                                                            * +
      |                                                            * |
  5ms +                                                           *  +
      |                                                           *  |
  4ms +                                                           *  +
      |                                                           *  |
  3ms +                                                          *   +
      |                                                          *   |
  2ms +                                              (midpoint) *    +
      |                                                  |    **     |
  1ms +                                                  v ***       +
      |             zfs_delay_scale ---------->     ********         |
    0 +-------------------------------------*********----------------+
      0%                    <- zfs_dirty_data_max ->               100%
.fi
.sp
Note that since the delay is added to the outstanding time remaining on the
most recent transaction, the delay is effectively the inverse of IOPS.
Here the midpoint of 500us translates to 2000 IOPS. The shape of the curve
was chosen such that small changes in the amount of accumulated dirty data
in the first 3/4 of the curve yield relatively small differences in the
amount of delay.
.sp
The effects can be easier to understand when the amount of delay is
represented on a log scale:
.sp
.nf
delay
100ms +-------------------------------------------------------------++
      +                                                              +
      |                                                              |
      +                                                             *+
 10ms +                                                             *+
      +                                                           ** +
      |                                              (midpoint)  **  |
      +                                                  |     **    +
  1ms +                                                  v ****      +
      +             zfs_delay_scale ---------->        *****         +
      |                                             ****             |
      +                                          ****                +
100us +                                        **                    +
      +                                       *                      +
      |                                      *                       |
      +                                     *                        +
 10us +                                     *                        +
      +                                                              +
      |                                                              |
      +                                                              +
      +--------------------------------------------------------------+
      0%                    <- zfs_dirty_data_max ->               100%
.fi
.sp
Note here that only as the amount of dirty data approaches its limit does
the delay start to increase rapidly. The goal of a properly tuned system
should be to keep the amount of dirty data out of that range by first
ensuring that the appropriate limits are set for the I/O scheduler to reach
optimal throughput on the backend storage, and then by changing the value
of \fBzfs_delay_scale\fR to increase the steepness of the curve.
